1. Field of the Invention
The invention relates to insulating resin powders useful for application to metal surfaces and to a method of forming agglomerated powder particles having improved coating properties and reduced dust generation during powder application.
2. Description of the Prior Art
Various methods of preparation of free-flowing powders are known, including suspension polymerization, spray drying, plasma spray and fluid bed processing. Types of powder formation for which these methods are particularly suitable are the formation of toner powders useful in imaging applications such as copiers and printers associated with computer equipment and formation of powders used to coat metal surfaces. Toner particles preferably are spherical particles having smooth surfaces; metal coating particles are preferably somewhat smooth, but not completely spherical.
The following powder formation methods are among those disclosed in the prior art. U.S. Pat. No. 4,345,015 discloses a method for forming spherical resin particles from irregularly shaped thermoplastic resin particles. Conductive carbon black either exists inside the resin particles or as a surface layer. A dispersion in a liquid carrier contains resin particles and silica particles to keep the resin particles separated. When the liquid is heated, the resin particles soften and adopt a spherical shape. Subsequent cooling, powder separation and drying yields a final toner product with such spherical shape.
German Patent DE 3533625 discloses a method of coating glass beads or bubbles with a synthetic resin powder. Essentially, a thermoplastic or thermosetting synthetic resin layer is deposited on the surface of the glass particles during heating of an intimate mixture of the glass particles and the resin. The coated particles provide lightweight molded structures suitable for further processing.
Chinese Patent CH 631635 discloses the formation of a spheroidal granulate by spray-drying of a composition of small particulates in a binder-containing solvent. Plasma flame treatment modifies the size thereof, producing free-flowing powders having desired particle size. Applications disclosed for the powders include metal surface coatings.
Canadian Patent CA 2005978 discloses attachment of a surface layer comprising a thermoset resin, preferably an epoxy resin mixture, to an inorganic or mineral filler such as quartz, mica, kaolin, etc. The coating process uses a fluid-bed mixer supplied with a combination of filler and thermoset resin powders. When the mixer is heated to a suitable temperature, the resin melts and deposits as a surface coating on the filler particles.
Japanese Patent JP 63319037 discloses surface modification of powders by means of a stream of hot air such that composite grains coated with fine particles form grains with a surface layer produced when the fine particles melt. Similar heated-air treatment of powders, revealed in Japanese patent JP 5011497, produces toner powder with less fine dust thereby improving surface charge stability and image quality. In this case, a precipitate, formed in a liquid medium, is isolated as a dry powder before heating. A stream of air operating for about one second at about 300.degree. C. supplies sufficient heat to form a toner powder of desired characteristics.
These prior art disclosures require significant amounts of heat in order to effectively process the powder. In most cases, the process equipment is heated along with the powder products. If means existed for heating the powder without significantly heating the process equipment, cost reduction and process efficiencies result. The current invention provides such a means for heat treating powders to give smooth agglomerated particles with rounded surface features.